IN DEPTH: The blade revolution

The supersize rotor blades that have come to dominate the wind turbine market in recent years are generally silhouetted alongside passenger jetliners to give a sense of their gigantic scale. Now it’s rockets.

Designer Blade Dynamics’ record-breaking 100-metre model — a slimline, modular concept on track to begin scaled prototype testing next year — will be almost as tall as the iconic Saturn V, the US spacecraft flown on the Apollo and Skylab missions between 1966 and 1973.

The comparison is perhaps doubly apt. Not only will the D100 blade be physically huge, it also has the potential to catapult the wind industry towards something like its own moon landing: the 10MW-plus turbine.

“Blade Dynamics is focusing entirely on delivering the next generation of blades through innovation that greatly reduces variation in manufacturing processes and makes blades more reliable,” says chief executive Pepe Carnevale.

“The fact is that real innovation is necessary if wind energy is to have a future. It is absolutely not the time for ‘boring’. It is the time to strive for economic, predictable and dependable. Scaling up conventional blades is what we perceive as risky.”

“Boring” is something the company could never be accused of being.

Its 100-metre “split blade” concept — underwritten by £15.5m ($24.9m) in funding from UK government-industry R&D body the Energy Technologies Institute’s Very Long Blade project — is based on a design innovation that jettisons conventional single-mould-based fabrication in favour of proprietary “non-mechanical” jointing techniques that bond 10-30-metre carbon-fibre sections into one lightweight, transportable, ultra-long unit.

The modular structure, high-performance tip and novel “corrugated” root configuration help create a structure about 40% lighter than a conventional glass-fibre model of the same length. Blade Dynamics’ calculations suggest that the blade has game-changing economics — annual output from a turbine fitted with the blades would be increased by around 15%.

“For the D100, the first half of last year was largely a phase of the project that looked at a whole raft of technology developments that were necessary to design this very large offshore blade — including joining technologies, advanced lightning systems, advanced aerodynamics, surface protection for higher tip speeds and so on, all of which was divided between our US and UK operations,” says Blade Dynamics senior technical manager David Cripps.

“The second half was dedicated to blade design work, and that included a great deal of analysis and cost modelling to determine the CoE [cost of energy] the blade could promise — to answer the question of whether this design was technically and commercially interesting.”

The root portion of a prototype 80-metre version has been constructed at the former Nasa facility in New Orleans where the Saturn V booster was built, while the tip has been manufactured 7,400km away at Blade Dynamics’ fabrication hall in Southampton, on the UK’s south coast. The root will shortly be en route to the National Renewable Energy Centre (Narec) in northeast England, where it will be mated to the tip and then trialled on the complex’s recently unveiled test bench.

“Made in America, built in Britain” to make best use of available fabrication hall space in southern England, the two sections of the blade have each been checked against a dummy tip or dummy root, to make sure tooling is accurate when they come to be fitted together.

“There is the issue of skilled personnel,” says Cripps. “We felt by splitting up the work we could reduce risk by not taking on a new facility here with a load of new people to build critical components.”

Dividing fabrication and assembly between two sites also resonates with Blade Dynamics’ supply-chain-centred manufacturing model. Its technology has been designed around quality-control systems employed in the automotive industry, with plans to introduce automation into parts manufacture and blade assembly “in the not-too-distant future”, in order to improve reliability, according to head of sales and marketing Theo Botha.

“The innate manufacturing variation of small, accurate parts is far lower than in larger components,” he says.

For an 80-metre blade, the modules can be up to 30 metres long, he explains, which is still “very large” for a composite component. “These are hardly small, but they are manageable and highly controllable.”

The root and tip sections are further segmented into parts “small enough that they are more easily controlled for quality but not so small that you have 100 pieces being assembled in a process that is overly arduous”,notes Cripps. “There is the additional advantage that these parts are of a complexity that they can be parcelled out as piece-work to smaller manufacturers without sacrificing reliability of the final product.”

Following an exhaustive programme of trials at Narec, it is expected that a maiden set of 80-metre blades will be manufactured for onshore road testing, before finally being scaled up to their full length for a future 10MW offshore machine. With no 10MW turbines up and turning, attaching heavyweight, 100-metre blades to a 6-8MW machine would be technically near-impossible and give scant insight into the CoE benefits to be had from the longer blade.

“You need a test model [of the blade] that allows you to gauge both the structural soundness of the blade on a rotor and its CoE reduction potential, so you need a length of unit that is compatible with the size of machine you’ll be testing on, you need a benchmark to work with,” says Cripps.

Building a demonstration blade for the 10MW-plus class “would be a big white elephant waiting for a turbine of this scale to be tested on”, he adds.

“There are significant design challenges that come in when you scale up from 80 to 100 metres in maintaining the torsion stiffness of a blade that is that long and slender, the tip-to-root response,” continues Cripps. “The structural requirements are substantially greater and we have made some interesting developments in overcoming these challenges in the 100-metre blade.”

Blade Dynamics’ blades feature more jointed sections than conventional moulded models, but use less adhesive because the joins are much thinner. The joins are also much wider, making them less susceptible to stress, and lighter. “Our joins are a strength, not a weakness,” underlines Cripps. “You just have to do it right.”

On a practical level, modularity means blade fabrication can be honed for greater efficiency, given the quality-control advantages that cannot be promised by fingertip searches of laminates laid up in conventional single-piece blade moulds.

And to speed progress in blade technology, segmentation would open up access to the tip, where “intelligence-adding” kit — embedded optic sensors that let the blade shed wind loads, or high-performance lightning conductors — could be wired in.

“People always seem to have a hang-up over the fact that we have joins in our blade — well, so do all blades out there. At worst, ours is no more risky than others’ [joining technology], and at best, because we concentrate to such an extent, our joins, being that ours is an openly modular design, have been worked into the fabrication process in a more integrated way,” says Cripps.

“Modularity started as a philosophy. Now we are starting to see the commercial spin-off — and the economic benefits match up well with the technological ones that come from quality control of mass-producible smaller sections, insurance against carbon wrinkles and so on, and this is particularly true in the very large offshore blades.”

Carnevale adds: “The fact is that real innovation is necessary if wind energy is to have a future. The opportunity demands more than linear, iterative evolution. Some OEMs are doing a fantastic job pushing forward, and now is the time to embrace the opportunity to lower the cost of wind dramatically using new technology.”